FIG. 11 shows an example of a conventional solenoid of this type. FIG. 11 is a schematic sectional view of a conventional solenoid.
This solenoid 100 is used as the driving means of a control valve, such as a spool valve for controlling, for example, hydraulic pressure. It is mounted in a liquid-tight fashion in a housing 200 filled with oil serving as the fluid through the intermediation of an O-ring 201. That is, a valve or the like (not shown) is mounted to the forward end of a rod 106 constituting the solenoid 200, and is driven in a state in which it is immersed in an oil (O) to thereby perform hydraulic pressure control or the like.
This solenoid 100 comprises a plunger 103 made of a magnetic material and slidably inserted into a bearing 102 provided in a plunger chamber P formed inside a hollow solenoid main body 101, a center post 104 formed of a magnetic material and arranged opposite to and coaxially with the plunger 103, a cap 105 which is provided on the opposite side of the center post 104 to regulate axial movement of the plunger 103 and which closes the plunger chamber P, and a rod 106 connected to the plunger 103.
The rod 106 is inserted and secured in a through-hole 103a extending along the center axis line of the plunger 103, and extends on the center post 104 side. Further, the center post 104 also has a through-hole 104a extending along the center axis line thereof, and the rod 106 is inserted into this through-hole 104a through the intermediation of a bearing so as to be capable of reciprocating such that its forward end portion protrudes outwardly.
And, as stated above, a valve or the like (not shown) is connected to the forward end of this rod 106, and is operated in accordance with the movement of the rod 106.
In this solenoid 100, normally, that is, when the solenoid main body 101 is not being energized, the plunger 103 is moved to the cap 105 side through the rod 106 due to the pressure of the external fluid such as oil or the force of a spring for returning the rod 106 in the valve or the like.
And, when the solenoid main body 101 is being energized, the plunger 103 is magnetically attracted to the center post 104, whereby the rod 106 connected to the plunger 103 moves to the left in the drawing to thereby drive the valve or the like.
Since the solenoid 100 is mounted in oil in a liquid-tight fashion, the oil may offer resistance when operating the plunger 103, thereby affecting the responsiveness of the plunger.
In order that the responsiveness of the plunger 103 may not be affected, there are provided oil holes 104b and 103b extending axially through the center post 104 and the plunger 103, respectively, whereby the oil from the exterior (P0) of the solenoid 100 flows through the oil hole 104b, the portion (P1) between the center post 104 and the plunger 103, and the oil hole 103b to the portion (P2) between the plunger 103 and the cap 105.
That is, in this case, the external oil moves as follows: P0→P1 and P1→P2 due to the movement of the plunger 103 (See the arrows in the drawing).
Although this construction helps to achieve an improvement in responsiveness, there still remains a problem in terms of durability due to abrasion or the like.
That is, as a result of wear of the valve or the like, a large amount of contaminants exist in the oil.
In particular, in the case where the flow passage as described above is formed, contaminants are likely to gather in the portion (P1) between the center post 104 and the plunger 103. This is because this portion constitutes a part of a magnetic path and is magnetized.
Thus, when the contaminants accumulated in this portion enter the portion where the plunger 103 slides, the plunger 103 is further worn, and the control characteristics can be adversely affected.
In view of this, the present applicant has filed a patent application regarding a technique to solve the above problem. This technique is disclosed in Japanese Patent Application Laid-Open No. Hei 9-89145.
This technique will be described with reference to FIG. 12. FIG. 12 is a schematic sectional view of a conventional solenoid.
As shown in FIG. 12, in this solenoid, the rod 206 is equipped with a through-hole 206a for directly guiding the external fluid (oil) to the plunger chamber P.
Due to this construction, the external fluid flows from the exterior (Q0) of the solenoid through the through-hole 206a, the plunger chamber P (Q1), and an oil hole provided in the plunger to the portion (Q2) between the center post and the plunger.
Thus, the contaminants do not easily reach the portion (Q2) between the center post and the plunger, thus solving the above problem.
However, the above-described prior-art technique involves the following problem:
It is true that, by providing a through-hole serving as a flow passage in the rod, it is possible to some degree to achieve an improvement in abrasion resistance. However, it can happen that some of the contaminants contained in the fluid directly guided to the plunger chamber get into the plunger sliding portion, resulting in a deterioration in abrasion resistance.
Thus, to further stabilize and improve the control characteristics, there is a demand for a further improvement in abrasion resistance.
Therefore, the present invention has been made with a view toward solving the above problem in the conventional art. It is an object of the present invention to provide a high-quality solenoid in which an improvement has been achieved in abrasion resistance.